T-Mobile is GSM like AT&T but would require different, AWS bands for 3G. They also offer "4G" over HSPA+, which iPhone 5 just might support. However, AT&T is trying to buy them, so they might get the iPhone that way sooner or later. Sprint is CDMA like Verizon but would need access to their network. They also offer "4G" over Wimax, which Apple will probably never support because LTE will be a much bigger, more global technology. (See our wireless networking glossary for what all those letters mean.)

If you could have only one for iPhone 5, T-Mobile or Sprint, which would it be?

I've been with T-Mobile U.S. for years (with my iPhone) and have to say that their customer service has been STELLAR. It's the best customer service I've had (and I've been with PacBell, Cingular, AT&T, and T-Mobile).
I'm already using the iPhone on T-Mobile right now.

T-Mobile has consistently gotten high marks for customer service, often ranking #1. Maybe you had a bad experience -- that happens -- but I've had 9 years of great experience with T-Mobile, and the overall ratings are consistent with great customer service.

Stupid buttons, meant to reply only.
Get her an unsubsidized iPhone and the cheaper data plan that only has 100MB of "4G" data and unlimited 2G data. The data is slow but if she wants an iPhone that badly then the data speeds probably aren't a primary concern. As pointed out above, there are people that do this already.

It should be on both. The iPhone should've been on all carriers since it's inception. Exclusivity like the iPhone had with AT&T is illegal in other countries. It's ridiculous and dwarfs sales. Limiting the consumer to one, bad carrier is a terrible decision. PlayStation did it again by choosing AT&T. There are many people who want the iPhone, but refuse to be locked into a carrier that they don't want. This argument is tired and shouldn't exist anymore. It's nothing but nonsense and frustration for the customers (or potential customers), especially since iOS has finally joined the world of modern mobile OSes.

Apple tried other carriers with the iPhone before it launched AT&T was the only us carrier that would give them a chance sprint tmobile and Verizon turned them down that's why AT&T locked them in for 4 years and now every carrier wants the iPhone because of the success it has brought AT&T and now Verizon

BOTH.
The current makeup and blatant disregard for what THE CUSTOMER WANTS by the current carriers AND their on-going attempt to stuff us into a 'plan', slave contract, Apple should do as the rumors speculate - UNLOCK IT and let the CUSTOMER CHOOSE !

As crazy as this sounds I don't think that T-Mobile deserves any profit from the iPhone. Their only goal in commercials now are to make the iPhone look bad. At first they said things like iPhone with AT&T is too slow. Then Apple added verizon but T-Mobile still said the iPhone was slow with AT&T as if it was the only choice. And it's not even their commercial idea, they just took the idea from Apple. I say they add sprint and let T-mobile suffer because I don't think there is a carrier on the planet that wouldn't want to gain from the iPhone's profit.

Ok my bad. But truthfully that just makes it worse. What will they say if sprint gets on the iPhone? Will they say everyone else is slower. My point is is that they think they are top dog because they have 4G on a phone, when the phone itself is nowhere close to as functional as the phone they are making look bad. Sprint is a great choice, not T-Mobile.